Iridium vs. copper
It's a fact. A spark always jumps from the sharpest edge of the center electrode. But the edges of copper center electrodes round off quickly, increasing the number of starter rope pulls. Iridium-enhanced spark plugs, long standard on late-model cars and trucks, are now available for lawn and garden equipment and motorsports applications. Iridium has the highest melting point of all the precious metals (4,229 degrees F), so it holds its sharp edge longer. Iridium plugs cost a few bucks more, but they provide faster starts and last much longer than conventional copper plugs.
Sharper Edge, Smaller-Diameter Electrode
Look at the difference between an iridium-enhanced spark plug and a conventional copper plug. The iridium-enhanced center electrode is much smaller (.6 mm vs. 2.5 mm) to concentrate the spark. Plus, it holds a sharp edge longer.